This invention relates to a structure for sealing an inner peripheral portion of a metallic diaphragm used in diaphragm valves, diaphragm type disk valves or the like, of a type installed in fluid transfer lines of semiconductor manufacturing plants, for example.
A structure disclosed in the U.S. Pat. No. 4,750,709, for example, is known for sealing an inner peripheral portion of a metallic diaphragm in a diaphragm valve and the like.
In such a prior art sealing structure, as shown in FIG. 5 of this application, an inner peripheral edge portion of a diaphragm 30 is welded to a seat holder 31 at a location W and the inner peripheral portion of the diaphragm 30 is pressed and held in an air-tight state between a metallic holding fitting 32 and the seat holder 31, thereby preventing leakage of fluid about the inner peripheral edge portion of the diaphragm 30.
In FIG. 5, the numeral 33 represents a body, 33a a valve chamber, 33b a valve seat, 33c a fluid passage, 34 a bonnet, 35 a seat, 36 a stem, and 36a a threaded portion formed at a lower edge portion of the stem.
In this structure for sealing the inner peripheral portion of a conventional metallic diaphragm, as shown in FIG. 6, because the inner peripheral portion of the diaphragm 30 is pressed and held between the annular metallic holding fitting 32 and the seat holder 31, with an entire top surface thereof being formed into a curved surface, an deep annular gap is formed between a lower surface of the diaphragm 30 and an upper surface of the seat holder 31. As a result, fluid has been easily trapped in this gap, and once fluid has been trapped in the gap, it has not been possible to easily remove it. Therefore, valves using this conventional sealing structure have had this shortcoming in terms of fluid displacement.
In addition, in this conventional sealing structure, because the inner peripheral portion of the diaphragm 30 is clamped between the holding fitting 32 and the seat holder 31 by a threaded structure, a thread has sometimes became loose over a long period of use, and a pressing and holding of the inner peripheral portion of the diaphragm 30 has sometimes been thereby loosened. As a result of the above, and the fact that the pressing and holding of the diaphragm 30 has been loosened, a large stress has been applied to the inner peripheral edge portion (weld zone) of the diaphragm 30, as the seat holder 31 moves upwardly and downwardly, thereby causing the inner peripheral edge portion of the diaphragm 30, and/or its weld, to easily crack. When the inner peripheral edge portion of the diaphragm 30 is cracked, reduced sealing performance results, which is extremely troublesome.
The present invention has been created to solve the above problems and an object of the present invention is therefore to provide a structure for sealing an inner peripheral portion of a metallic diaphragm which eliminates the trapping of fluid, forestalls cracks at the inner peripheral portion of the diaphragm, and demonstrates excellent sealing performance.